The present invention relates to digital video generators, and more particularly to a digital video generator for providing smooth scrolling of video images on a video display.
There are many applications where a video image is processed in digital form prior to display. Such applications include character generators in the television industry for special effects and scrolling of credits. More recently this processing also is being applied to teleprompter systems where a speaker reads lines from a television display while text is scrolled across the face of the display.
As an example, taken from the field of teleprompter systems, a person stands or sits in front of a television camera and/or a live audience and looks through a beam splitter. Reflected in the beam splitter is a television monitor upon which is displayed text to be read by that person. As the text is read, it scrolls up off the top of the monitor screen while new text appears at the bottom.
Traditionally, the text image is generated by typing the text on white paper and placing successive pages on a conveyor belt. The conveyor belt brings the pages past the lens of a television camera and appropriate light sources. The resulting television picture is sent to the television monitor, which monitor typically is modified to reverse the image top-to-bottom so that the image is upright when viewed in the beam splitter. The conveyor belt is controlled with a rheostat for speed and a switch for direction, and two people are required to operate the system--one to control the conveyor belt speed, adapting it to the reader's speaking rate, and the other to feed the text pages onto the belt.
There have been some attempts to build computer-based teleprompter systems by combining the features of a word processor with the electronic generation of the scrolled text. These systems suffer from poor character quality and from irregular scrolling.
First, a digital character has unnaturally sharp edges, which is objectionable to the human eye, as opposed to an analog character from a television camera which has gray corners and edges. This problem, known generally as "aliasing", is well understood. The dots, squares or pixels (picture elements) which make up a digital character are either "on" or "off". The classic technique for anti-aliasing is to filter the data in such a way as to simulate the response of a non-discrete system, such as the television camera, which blurs the edges of the character.
Second, current digital systems do not scroll smoothly at arbitrary scrolling speeds. In non-discrete systems the blurring of the character image makes possible the appearance of partial video line movement. However, a digital system, which turns dots either "on" or "off", moves the image a whole line at a time or not at all. This produces a jumpy scrolling motion which is a temporal aliasing problem. This problem exists in the scrolling of any digital video image, not just in teleprompter systems.
What is desired is a digital video generator which provides good character quality and smooth scrolling characteristics.